1. Field of the Invention
The present invention is directed to a correlation type autofocus chip for a camera which provides reference levels for a conversion operation from within the chip and, more particularly, to a system which uses a full well signal from bright diodes of a charge coupled device (CCD) sensor to set a high reference level and the reset level experienced by the sensor as the low reference level for the conversion operation.
2. Description of the Related Art
Correlation type autofocus systems require a linear sensor, a method of sensing the light level in the scene and means of correlating the image falling on the left hand of the image sensor to that falling on the right half. Typically the signal from each pixel of the sensor is digitized to allow the correlation to be performed in the digital domain, either by on-chip signal processing circuitry or the on-camera microprocessor. Examples of correlation type autofocus systems are the Fuji Electric Company LTD part#FB6206 and those described in U.S. Pat. Nos. Re 32,886, 4,831,405, 4,873,543 and 4,878,079. These types of autofocus systems are passive systems which means that they depend on the light in the scene for their operation. Since the scene light level is not known before hand, the system must measure the light level and adjust the integration time to produce an adequate signal level. For a maximum signal to noise ratio it is necessary to know very accurately the full well signal level (the electron saturation level) that a CCD linear sensor can produce. During operation of the conventional autofocus system, the signal charge, in the form of electrons stored in the photodiodes, is transferred to a storage register, clocked out of the storage register, converted into a voltage and the voltage is applied to the converter to produce a digital value for the light sensed by the diodes. In this conventional circuit the high and low voltage references provided to the D/A converter are supplied from an external source, typically the voltage reference from a microprocessor that performs the correlation processing. In the Fuji chip previously discussed, the reference is determined by counting. In all cases, however, each camera must be individually calibrated. These voltage sources provide voltages independent of any changes that take place in the chip. For example, as a camera moves from indoors to outdoors, temperature fluctuations occur which change the characteristics not only of the photodiodes but also of the amplifiers, etc. that convert the charge into a voltage and apply it to the converter. The manufacturing process also results in variations. All of these variations result in only an approximation of the full well level and a variation around 1 volt being supplied to the converter. These reference voltage variations cause variations in the converted values such as producing a different converted value when the light conditions are the same because the chip has experienced an environmental change. As a result, the photographic media would normally experience differences in the precision of the focus from picture to picture. In conventional cameras, digital signal processing routines are used to attempt to correct for these variations. As described in U.S. Patent No. 4,831,405 the CCD output is measured and a microprocessor adjusts amplifier gain to perform such a correction.